Aztreonam is a monobactam antibiotic disclosed in U.S. Pat. No. 4,775,670, which is incorporated by reference herein in its entirety. Aztreonam has the chemical name (Z)-2-[[[(2-amino-4-thiazolyl)[[(2S,3S)-2-methyl-4-oxo-1-sulfo-3-azetidinyl]carbamoyl]methylene]amino]oxy]-2-methylpropionic acid. Aztreonam is also known as [3S-[3α(Z),4β]]-3-[-[(2-amino-4-thiazolyl)[(1-carboxy-1-methylethoxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidinesulfonic acid and (2S,3S)-3-[[2-[2-amino-4-thiazolyl]-(Z)-2-[(1-carboxy-1-methylethoxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidine-1-sulfonic acid. Aztreonam has the structure:

U.S. Pat. No. 4,775,670 discloses a process for making Aztreonam and pharmaceutically acceptable salts thereof. However, U.S. Pat. No. 4,775,670 does not teach how to prepare salts of Aztreonam with amines or amino acids.
Applicants encountered unexpected difficulties when trying to prepare salts of Aztreonam with amines and amino acids by dissolution of the acid and base in a solvent and precipitation of the salt. In the majority of experiments an oil, which was impossible to crystallize and which decomposed very rapidly, was obtained.
Applicants have discovered methods that enable the preparation of a solid, stable Aztreonam L-lysine salt.